This invention relates to optical waveguides, and more particularly to optical waveguides that can be used in optical recording and thermally assisted magnetic recording. Heat assisted magnetic recording (HAMR) generally refers to the concept of locally heating a recording medium to reduce the coercivity of the recording medium so that an applied magnetic writing field can more easily direct the magnetization of the recording medium during the temporary magnetic softening of the recording medium caused by the heat source. Heat assisted magnetic recording allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature to assure sufficient thermal stability.
Heat assisted magnetic recording requires an efficient technique for delivering large amounts of light power to the recording medium confined to spots of, for example, 50 nm or less. Recent designs of HAMR recording heads include a thin film waveguide on an AlTiC slider to guide light to a storage medium for localized heating of the storage medium. To launch light into the waveguide, a grating coupler can be used. Due to the limited size of the slider, the size of the incident beam is only ˜50 μm. At this beam size and with conventional symmetric surface corrugation grating couplers, coupling efficiency from the incident beam to the waveguide is low (<20%).
In thermally assisted magnetic recording, information bits are recorded on a storage layer at elevated temperatures, and the heating area in the storage layer determines the data bit dimension. In one approach, a beam of light is condensed to a small optical spot onto the recorded media by a solid immersion mirror fabricated on a planar waveguide. In this approach, light is coupled into the waveguide by a diffraction grating, which is optimized to yield good coupling efficiency for a given incident beam. The range of angle of incidence at the half maximum of coupling efficiency is less than 0.7° for an incident Gaussian beam of 50 μm. With such a low acceptance angle of incidence, it is difficult to achieve a high coupling efficiency in a practical HAMR slider.
There is a need for an apparatus that provides an increased acceptance angle of incidence to improve the efficiency of coupling an electromagnetic wave into a waveguide.